Liquid crystal displays (LCDs) are utilized in a great variety of direct view type displays and virtual image displays. Examples of direct view displays in which LCDs are utilized include digital watches, telephones, lap-top computers and the like. Examples of virtual image displays in which LCDs are utilized include camcorder viewfinders, head-mounted displays, or virtual displays in portable electronic equipment, such as portable communications equipment.
Liquid crystal displays have three basic modes: transmissive, reflective and transflective. In transmissive mode, the LCD pixels are illuminated from behind (i.e. opposite the viewer) typically using a cold cathode fluorescent lamp (CCFL) or LED. Transmissive LCDs offer the best performance under lighting conditions varying from complete darkness to an office environment. In very bright outdoor environments they tend to “wash out” unless they have high brightness backlights. Transmissive LCDs offer a wide color range, however, they typically require a color filter for full color display.
In reflective LCDs, the pixels are illuminated from the “front” (i.e. the same side as the viewer). Reflective LCD pixels reflect incident light originating from the ambient environment or from a frontlight. Reflective LCDs offer very low power consumption (especially without a front light) and are often used in small portable devices such as handheld games, PDAs or other portable instrumentation. Reflective LCDs offer the best performance under lighting conditions typical of office environments and brighter. Under dim lighting conditions, reflective LCDs typically require a frontlight.
A “transflective” display is capable of operating in both transmissive and reflective modes. Examples of transflective displays are provided in Lueder et al., “The Combination of a Transflective FLCD for Daytime Use With An OLED for Darkness,” 2000 Society for Information Display (SID) Symposium Digest, 1025-1027, and Lee et al., “Development of the new structure of transflective LCD,” Korean Information Display Society (KIDS) international Meeting on Information Display Digest (IMID) 2001, Session A8.3. In the reflective mode, ambient light reflected back towards the viewer's eye by elements within the display provides an image to a viewer. A reflective mode may be particularly advantageous for certain applications, such as a laptop display that may be used outside during daylight, which might obscure the images generated by a display that does not have a reflective mode. A transmissive mode is advantageous for such a display in a different environment, for example when there is less ambient light. In general, during operation in the transmissive mode, transflective liquid crystal devices are illuminated with a relatively large, separately mounted light source, preferably from the rear (back-lighting), so that most of the light travels directly through the liquid crystal and outwardly to the eye or eyes of a viewer. To provide a suitable amount of light or luminance at the outlet, the LCD requires a relatively bright, and typically large, backlight source. Generally, the result is to produce a relatively large and cumbersome package, usually with several discrete components that may be difficult to incorporate in portable electronic equipment, or the like.
Typically, red, green and blue light emitting devices (LEDs) have been very important to the development of a small LCD backlight. In the past, fluorescent lamps were used to light these small LCDs. Fluorescent lamps are suitable for lighting large direct view LCDs, but are a problem at the smaller sizes including virtual image displays. As a general rule, as the length and diameter of the lamps decrease, the efficiency also decreases, making small lamps very power hungry. In addition, the individual red, green and blue emission from the fluorescent lamps cannot be controlled individually, thus field sequential color is difficult. For example, in portable electronic devices such as telephones, two-way radios, pagers, etc. the displays are limited to a few alpha-numeric digits viewed as a direct view image. Generally, if a small portable device is desired, the display must be reduced to a very small number of digits, since the size of the display dictates the minimum size of the device into which it is integrated.
U.S. Pat. No. 5,965,907 suggests the use of a stacked OLED backlight capable of field sequential color in a transmissive LCD display, however, to meet the demands of a transflective display and still maintain small size, there is a need to improve the efficiency of OLEDs used for that purpose. A large portion of the light generated from the active regions of a typical OLED is totally-internally-reflected and lost before escaping the device due to refractive index mismatches in the device layers. Moreover, in a typical transflective display, a large amount of the light that is emitted from the backlight in the transmissive mode is absorbed by a partially transmissive/partially reflective “transflective” layer located between the backlight and the viewer. In addition, the partially transmissive/partially reflective transflective layer compromises reflectivity in the reflective mode because of the necessity to accommodate operation in both modes. Because of the many loss mechanisms present, providing sufficient illumination in the transmissive mode while at the same time optimizing the reflective mode in transflective display, remains a significant problem.